Double-stranded RNA has been shown to attenuate specific gene expression in C. elegans, Drosophila and Trypanosoma brucei (M. Montgomery, et al., Proc. Natl. Acad. Sci. U.S.A. 95, 15502-15507 (1998); J. Kennerdell et al., Cell 95, 1017-1026 (1998); H. Ngo et al., Proc. Natl. Acad. Sci. U.S.A. 95, 14687-14692 (1998)). The types of genes attenuated in these invertebrates include some encoding transcription factors and others that encode growth factor receptors. There is also evidence that double-stranded RNA may effectively silence gene expression in plants (M. Wassenegger et al., Plant. Mol. Biol. 37, 349-362 (1998); P. Watergiyse et al., Proc. Natl. Acad. Sci. U.S.A. 95, 13959-13964 (1998)). Application of this method to vertebrates would be an extremely useful tool for the study of vertebrate developmental genetics and has numerous medical implications as well, however researchers have heretofore been unable to obtain successful gene silencing in vertebrates using this method.
A definitive mechanism through which double-stranded RNA effects gene silencing remains has not been identified (M. Montgomery et al., Trends Genet. 14, 255-258 (1998)). Recently, Montgomery et al. reported that double-stranded RNA induces specific RNA degradation in nematodes (Proc. Natl. Acad. Sci. U.S.A. 95, 15502-15507 (1998)). This conclusion was based upon the fact that DNA sequences in the targeted regions of the gene were not altered and that 100% of the F2 generation reverted to the wild type phenotype. In addition, C. elegans has a unique genetic organization. Genes in this animal are organized in operons in which a single promoter controls expression of a number of genes. They showed that the double-stranded RNA affects only expression of the targeted gene. In contrast, however, others have observed heritable effects of double-stranded RNA on the expression of a number of genes in C. elegans, suggesting that more than one mechanism may be involved in double-stranded RNA-mediated inhibition of gene activity (H. Tahara, Science 28, 431-432 (1998)).
In transgenic plants, co-suppression of gene expression can be mediated through rapid degradation of the mRNA produced by the targeted gene (D. Smyth, Curr. Biol. 7, R793-795 (1997)). Others have shown that double-stranded RNA-dependent sequence-specific methylation may mediate the long-term effects of co-suppression in plants. Such a methylase may also be dependent on transcription of the targeted sequence since double-stranded RNA targeted to promoter regions in nematode had no apparent effect on transcription.